The present application relates to electric power conversion, and more particularly to circuits, methods and systems which use a smart current-modulating architecture, and most particularly to the detection of instants when power-carrying current crosses zero in power converters which use such an architecture.
Note that the points discussed below may reflect the hindsight gained from the disclosed inventions, and are not necessarily admitted to be prior art.
Numerous techniques have been proposed for electronic conversion of electric power from one form into another. A technique in common commercial usage for operating three phase induction motors at variable frequency and voltage off of fixed frequency and voltage utility power is the AC-DC-AC technique of the input diode bridge, DC-link capacitor, and the output active switch bridge, under pulse-width modulation (PWM) control. This motor drive technique (“standard drive”) results in compact and low-cost motor drives, since no magnetic components are required and only six active switches are needed.
A number of difficulties exist with the standard drive, however. The input current, while nominally in phase with the input voltage, is typically drawn in pulses. These pulses cause increased electric losses in the entire electrical distribution system. The pulses also cause higher losses in the DC link capacitor. These losses reduce the efficiency of the drive, and also lessen the useful life of the DC link capacitor (commonly an aluminum electrolytic type), which has a limited life in any case. If the impedance of the source power is too low, the pulses may become so large as to be unmanageable, in which case it is necessary to add reactance in the input lines, which increases losses, size, cost, and weight of the drive. Also, the voltage available for the output section is reduced, which may lead to loss-producing harmonics or lower-than-design voltage on the output waveform when full power, full speed motor operation is called for.
The term “converter” is sometimes used to refer specifically to DC-to-DC converters, as distinct from DC-AC “inverters” and AC-AC “cycloconverters.” However, in the present application the word converter is used more generally, to refer to all of these types and more, and especially to converters using a current-modulating architecture.
A new kind of current-modulating power converter was disclosed in U.S. Pat. No. 7,778,045, titled “Universal power conversion methods,” the disclosure of which is incorporated by reference into the present application in its entirety. This patent describes a bidirectional (or multidirectional) power converter which draws power from the utility lines with low harmonics and unity power factor, is capable of operating with full output voltage even with reduced input voltage, allows operation of its switches with low stress during turn-off and turn-on, is inherently immune to line faults, produces voltage and current output waveforms with low harmonics and no common mode offsets while accommodating all power factors over the full output frequency range, operates with high efficiency, and which does so at a reasonable cost in a compact, light-weight package. This architecture has subsequently been referred to as a “current-modulating” architecture. Bidirectional power switches are used to provide a full bipolar (reversible) connection from each of multiple ports to an LC link reactance. Optimal timing of the switching, in such an architecture, is easier if the instantaneous current levels in the link inductor can be sensed accurately.
In previous implementations, estimation of current-zero-crossing can be implemented, for example, by shutting all switches, monitoring the voltage on the LC combination, and estimating that the peak voltage (due to the resonance of the inductor and capacitor) would correspond to the zero of the current. However, it was found that errors in peak detection would sometimes occur. These errors in peak detection may have been due to the electrically noisy environment in which power conversion circuits are often operated, but for whatever reason, such errors would sometimes occur. The consequence of such an error would be an error in switching optimization, which would propagate forward to further disturb efficient operation.
The present application discloses inventions which not only work synergistically with such architectures, but also have other advantageous uses.